Lumber from many tree species lacks durability and frequently has inferior physical properties. These deficiencies are more likely to occur in lumber extracted from man-made plantation forests. Since durability and enhanced physical properties can be required it is typical for lumber processors to alter lumber properties.
It is well known to those versed in the art that these deficiencies can be remedied to a greater or lesser extent by impregnation of the lumber with preservatives, polymers and the like. Such impregnation processes have been used for many decades and most frequently involves impregnation with treating fluids.
A relatively modern trend is to treat lumber in its final ready to use form. This eliminates any contaminated waste streams (saw dust, shavings and the like) which would otherwise occur during subsequent processing of lumber treated in crude form.
When treating lumber it is preferable to have the lumber already dry, that is, with its cells empty. This is because space is required for the incoming treating fluid.
Typically lumber is treated with either waterborne preservatives, or with solvent fluids based on non-polar organic solvents such as white spirits or Stoddard solvents (used in LOSP processes). Both processes are similar in that variations of vacuum and pressure are used.
A deficiency of known waterborne processes is that substantial uptakes are required to achieve full penetration. This in part is due to rewetting of the cell wall and to adsorption of water into or onto the cell wall. Thus to overcome this adsorption and ensure full penetration, uptakes can vary from 150 litres per cubic metre to 600 litres per cubic metre depending on the process used. Treatment with waterborne solutions causes swelling which is undesirable. Once treated, the lumber must be re-dried and this is costly. However waterborne processes do enable use of inexpensive well proven inorganic biocides.
A significant issue arises when using waterborne processes. Because the preservatives are necessarily soluble in water, they remain mobile for some time subsequent to treatment. That is they offer the potential for elution into the environment when in contact with ground water or when exposed to rain, with the potential hazard that might create. Modern processes can use a heating step wherein the interaction between the preservatives and the wood is hastened. This is time consuming, requires additional plant and a means of energy to raise the substrate temperature, and because the heat source is typically steam or hot water, waste streams contaminated with heavy metals result.
LOSP processes using non-polar organic solvents overcome the swelling problem and have quite low uptakes of around 30 to 40 litres per cubic metre. This is because there is no significant interaction between the solvent and the cell wall. That is, the solvent is non-polar and does not interact with or adsorb onto cellulose or lignin, which are polar. Uptakes can be as low as 30 to 50 litres per cubic metre. Drying in the normal sense is not required although the solvent must be allowed to evaporate. Whilst this process is effective for treating dry lumber, when using known methods eventually all the solvent escapes into the atmosphere thus becoming an environmental issue. Further, the solvent is manufactured from petroleum feedstocks thus is not a renewable resource and is subject to significant price variations.
According to traditional LOSP processes the solvent remains in the lumber after treatment. As LOSP solvents are expensive, using new solvent for every treatment is very costly. Further, the solvent contributes a serious environmental impact because the wood continues to release greenhouse gases and can adversely affect the health of persons living or working near the treated lumber. However, if the solvent was to be recovered and recycled for re-use, these issues could be minimised.
Many processes are known for the impregnation of lumber. These processes are adequately described in “industrial Timber Preservation”, 1979, J G Wilkinson, Associated Business Press.
Several such processes include those of: 1) Rueping: Pre-pressure with gas followed by pressure with preservative or chemical solution; 2) Lowry: Pressure impregnation with preservative or chemical solution; and 3) Bethel: Vacuum followed by pressure impregnation with preservative or chemical solution.
to The Rueping process applies pre-pressure with gas prior to treatment with preservative fluids. This pre-pressure with gas fills the cells with a compressible medium such that after treatment with fluid the gas will expand forcing out any surplus fluid. However this can result in ongoing kickback of preservative contaminated fluid which may be hazardous and which kickback fluid may contain extractives which will interfere with preservative chemistry.
The Rueping and Lowry processes retain gas within the void spaces within the substrate. Thus, the impregnation process requires pumps to force fluid into the substrate against the back pressure of the gases in the voids.
The Bethel process removes all gases from the cells by application of a vacuum which cells then become completely filled with preservative fluid. When using aqueous fluid this method has the disadvantage that lumber is completely filled which can not be sucked out again. Accordingly, the lumber takes considerable time to dry.
LOSP preservatives use a solvent known as a Stoddard solvent, otherwise commonly known as aliphatic white spirits or mineral spirits. LOSP processes may also use chlorinated hydrocarbons or low molecular weight aliphatic hydrocarbons as a solvent. The modern versions of this are refined to remove aromatic compounds to improve odour and reduce toxicity however this reduces the solubility of many biocides in the solvent and thus may be counterproductive. Moreover, impregnation processes used to apply LOSP formulations have been developed and refined to limit the amount of solvent transferred to the wood whilst ensuring substantial penetration. An example of this would be the “Double-Vacuum” process, wherein the wood is evacuated and then flooded with preservative, the vacuum is released to atmospheric pressure for a short time, then the preservative is transferred away from the wood and a second vacuum is applied to remove excess preservative.
Despite these improvements, costs continue to escalate and because of environmental concerns there is a growing trend away from products using LOSP preservatives. However because re-drying of the substrate is not required there still exists a potential market, particularly if any residual solvent could be recovered and recycled.
Stoddard solvents are highly flammable and therefore appropriate plant design and operating procedures must be used to minimise potential adverse consequences. Alternative organic solvents are available but since these are either costly or toxic they are not favoured.
Because lumber has substantial voids within which the preservative can be transferred, whether for waterborne preservatives or LOSP preservatives, the lumber must be substantially dry. LOSP preservative processes dictate that the lumber must be dried to its final moisture content, that is, around twelve to fifteen per cent on a mass basis.
Lumber for waterborne processes can have greater moisture content, that is, above fibre saturation. Fibre saturation occurs at around thirty to forty per cent moisture content on a dry mass basis for softwoods. Thus whilst LOSP treated lumber is still “dry” after treatment and requires no re-drying in the traditional sense, it is usually still an expensive process because it wastes significant volumes of solvents (VOC's—volatile organic compounds). The waterborne process allows for higher pre-treatment moisture content but still suffers the expensive re-drying process.
Drying of wet lumber can be undertaken by a number of means including RF vacuum drying. This can be very rapid if using the likes of high temperature kiln drying but is slow using RF systems because of the energy limitations of RF equipment.
The expense and inefficiency of drying using RF vacuum processes from green wood, that is around 150 per cent moisture content on a dry mass basis, for softwoods is problematic.
When the lumber is nearly dry (i.e at or around fibre saturation, which is 30 to 40 per cent for softwoods), RF drying is more effective and can offer benefits. RF energy can be applied to organic substrates including lumber, and this RF energy impacts directly with, and below fibre saturation can be absorbed by, the bound water. Because RF energy can penetrate readily throughout the substrate, energy flow is rapid. However the absorption of
RF energy depends on a material or compound within the substrate having the ability to absorb that energy.
Dielectric loss is electric energy that is converted into heat in a dielectric subjected to a varying electric field. It is also known as dielectric absorption. Polar compounds have a high dielectric loss. This is because they act as dipoles which interact with the varying electric field. Non-polar compounds, such as n-hexane or Stoddard solvent, have very little interaction with a varying electric field.
The dielectric loss of the material or compound within the substrate has a significant effect on the ability of the material or compound to absorb RF energy. Materials with low dielectric loss, such as the solvents used in traditional LOSP solvent system processes, will absorb little energy. Whereas a material with high dielectric loss, such as water and highly polar solvents including DMSO, N-methyl pyrrolidone and the like or glycols such as ethylene or propylene glycol or glycerol, glycol ethers, ketones, N-methylpyrrolidone, dimethyl sulphoxide, dimethylformamide will readily absorb the energy.
It is also important to consider the effects of RF energy on the substrate if that substrate is already substantially dry. For example if the lumber prior to treatment is at equilibrium moisture content, say between 12 and 15 per cent, RF heating will reduce the moisture content further. This could cause shrinkage, resin bleed, possibly checking, and if not controlled can cause degradation of appearance and strength. Resin bleed typically occurs if the temperature increases above 55 Celsius.
More recently practitioners have begun using aqueous emulsions of biocides to eliminate
Stoddard solvents and the consequences they create. This can reduce costs. However, aqueous emulsions cause swelling of the wood and grain lifting. These degrade appearance and can necessitate re-drying. This can result in the additional cost of re-drying, which can also cause distortion of the wood.
A method that could eliminate the cost and environmental impact of using Stoddard solvents and yet not result in swelling might offer a valuable alternative.